This invention relates to magnetic ballast for fluorescent lamps and in particular, to a new and improved ballast which has near unity power factor with low harmonic content and improved efficiency.
In the past, magnetic ballast circuits have been of three types: inductive, capacitive and resistive. Since a fluorescent lamp tends to have a high impedance and low current before ignition, and the reverse thereafter, it has been conventional to employ an impedance in series with the lamp to provide a high starting voltage and a reduced operating current.
In the pre-heat inductive circuit, the lamp is typically connected in series with an inductor and energized from two opposing filament terminals. The other two terminals are connected to a starter, typically a gas-tube-operated contactor and capacitor in parallel. When a voltage is first applied, the lamp filaments are cool and the starter gas-tube ignites at a lower voltage than the lamp. As the gas-tube warms, its built-in contactors close; filament current flows in the lamp and the inductor is energized. The closing of the contactors removes the voltage from the gas-tube, which cools, opening the contactors, which then cycle on and off. Each time that the contactors open, the energy stored in the series inductor tends to produce a voltage spike across the lamp, whose breakdown voltage with heated filaments is lower than that of the gas-tube. When the lamp ignites, the gas-tube extinguishers and its contact cycle ceases. The parallel capacitor in the starter is used to suppress electromagnetic effects associated with transients generated in the system.
This form of uncorrected magnetic ballast has a low power factor (typically 0.55) since the current passing through the inductor lags the applied voltage. Also it has appreciable ohmic loses in any practical size and cost configuration. By way of example, 3-5 watts in a typical 20 watt lamp ballast.
The inductor operating region is typically such as to be in partial saturation at maximum pre-heat current. The swinging choke characteristics are of critical importance. Of three inductors, all testing the same inductance and resistance on a meter, one could operate properly as a ballast; the second could fail to light the lamp; and the third could blow out the lamp filaments.
Series capacitive ballasting is closely related to inductive ballasting with the current leading instead of lagging, but with increased higher-frequency current components. It tends to be more efficient in operation with little ohmic loss, but does not have the desirable igniter starting characteristic of an inductive ballast with contactors. A combination of leading and lagging ballasts may be used for lamp pairs, thereby correcting power factor, but not necessarily correcting waveform.
Series resistive ballasting has good power factor and waveform, but poor efficiency, and is not considered a viable alternative.
A power factor corrected ballast using a capacitor across the input line to tune out the series inductive reactance at 60 hertz has been tested. While this configuration corrected the power factor, it raised the third harmonic current from about 8% to 12%.
Another type of power factor correction that is commonly used in rapid-start magnetic ballasts involves the use of a series capacitor to resonate with the inductor at 60 hertz. The current drawn in such a resonant circuit is typically a distorted sine wave, and would not be likely to meet stringent harmonic requirements.
It is an object of the present invention to provide a new and improved magnetic ballast of the series inductive type which will have a high power factor and a low harmonic content, as well as improved efficiency. One specific circuit of the present invention exhibits a power factor greater than 0.95 with a harmonic content of less than 5%, and power savings in comparison with the conventional single series inductance in the order of 10%.
Other objects, advantages, features and results will more fully appear in the course of the following description.